A heated battle between union leaders and New York City Mayor Michael Bloomberg over grading educators offers one of the defining issues in the race to succeed him.

As a result, five candidates -- all looking to change at least some school policies after Bloomberg leaves office Dec. 31 -- gave varied reactions at a mayoral forum Wednesday sponsored by the city principals' union.

The labor impasse has cost the city $250 million in state aid for which an evaluation deal was required by Jan. 17 to qualify. This week, with even more funding on the line, Bloomberg rationalized that the fight will prove worthwhile if it results in a tool for removing incompetent teachers.

In earlier talks, the mayor rejected a two-year sunset date for the evaluation system after which its details can be renegotiated. Other school districts around the state agreed to such a provision and thus met the deadline.

The issue also has put Bloomberg and Gov. Andrew M. Cuomo on different pages, with the lame-duck mayor saying most other districts acceded to bad deals just to salvage their share of the state's 4 percent increase in school aid. And Bloomberg said the state Education Department "accepted the plans they knew were total frauds."

Cuomo now says he'd step in and impose a system if necessary. Former Comptroller William C. Thompson Jr., a Democratic candidate, noted at Wednesday's forum that the state had already set a "blueprint" for a deal.

"For the city not to come to an agreement on evaluations with the unions is derelict," said Thompson, who chaired the city Board of Education years before the sprawling urban school district came under mayoral control. "It is astonishing. The fact that we have lost at least $250 million is outrageous. It's ridiculous."

Lhota said Thursday through a spokeswoman that the UFT, driven by its political agenda, "has resorted to personal attacks on Mayor Bloomberg and has acted irresponsibly by refusing to negotiate in good faith."

By contrast, Council Speaker Christine Quinn (D-Manhattan) seemed to walk a line discussing evaluations the other night. "We have to have an evaluation system. . . . The first thing is, everyone needs to get back into the room, lock themselves in the room if they have to."

On Tuesday, GOP candidate John Catsimatidis indicated after announcing his candidacy that the parties should have worked harder to reach an accord.

With evaluation the talk of the moment in school politics everywhere, wider views were solicited from the candidates at the Council of School Supervisors and Administrators event. Republican candidate Tom Allon, for one, said "it's not about some convoluted system" -- and voiced the view that "a good principal walking from class to class knows who the good teachers are."

Comptroller John Liu backed peer review as a key part in evaluations. Public Advocate Bill de Blasio said, "What I don't get is, this is focused on the notion that the biggest problem facing us is some percentage of teachers that are not measuring up. I believe that's a very small percentage."

He added: "We are already focused way too much on standardized tests in the school system."